Raymarine Guide to Fishfinder Echosounder Transducers

When trying to determine which transducer is best for you, youll need to consider the following variables:

What material the transducer housing is made of, based on boat hull composition
How the transducer should be mounted on the boat: in - the hull, through the hull, or on the transom.
What you want to see displayed: depth, speed, temperature, or a combination.
Determine what material the transducer should be composed of.

Plastic housings are recommended for fiberglass or metal hulls.

Bronze housings are recommended for fiberglass or wood hulls. Bronze is preferable to plastic for wooden hulls because the expansion of wood could damage a plastic transducer and cause a leak. Installation of a bronze housing in a metal hull requires an insulating fairing. A metal housing should NOT be installed in a vessel with a positive ground system.

Stainless Steel housings are recommended for steel or aluminum hulls.

Determine the type of transducer mounting.

In-Hull Transducers

In-hull (a.k.a. shoot-through) transducers are epoxied directly to the inside of the hull. These are only used in fiberglass hulls. In-hulls will not work with wooden aluminum, wood, or steel hulls, or in foam sandwich or hulls that have air pockets. Any wood, metal, or foam reinforcement must be removed from the inside of the hull.

With an in-hull transducer, the signal is transmitted and received through the hull of the boat. As a result, there is a small loss of sonar performance. In other words, you wont be able to read as deep or detect fish as well with an in-hull transducer as with one thats transom mounted or thru-hull mounted.

Thru-Hull Transducers

Thru-hull transducers are mounted through a hole drilled in the bottom of the boat and protrude directly into the water. This type of transducer generally provides the best performance.

Thru-hulls are recommended for displacement hulls and boats with straight-shaft inboard engines. Youll also need a fairing block that allows the transducer to be mounted properly. Thru-hull transducers must be installed with a fairing to ensure proper alignment and a secure fit.

Transom Mount Transducers

As the name implies, transom mount transducers are installed on the boats transom, directly in the water and typically sticking a little below the hull. Transom mounts are composed of plastic and tend to be less expensive than other transducers.

Transom mount transducers are recommended for planing hulls of less than 27 feet (8 meters), such as personal watercraft and powerboats with outboard, inboard-outboard and jet drives. They are not recommended for large or twin screw inboard boats because aerated water from the propeller reduces performance. They are also not recommended for operation at very high speeds.

Transducer Specifications

Power refers to the strength with which the transducer sends the sonar â€œpingâ€, expressed as watts RMS. Higher power increases your chances of getting a return echo in deep water or poor water conditions. It also lets you see better detail, such as bait fish and structure. Generally, the more power you have, the deeper you can reach and the easier it is to separate echoes returning from fish and bottom structure from all the other noises the transducer detects.

Frequency

The accuracy with which your fishfinder detects bottom and other objects is also determined by the frequency selected for the depth you are viewing. Raymarine depth transducers can be tuned to two different frequencies: 50 kHz (low) or 200 kHz (high).

200kHz

200 kHz works best in water under 200 feet/60 meters and when you need to get an accurate reading while moving at faster speeds. High frequencies give you greater detail to detect very small objects but over a smaller portion of water. High frequencies typically show less noise and fewer undesired echoes while showing better target definition.

50 kHz

For deep water, 50 kHz is preferred. This is because water absorbs sound waves at a slower rate for low frequencies and the signal can travel farther before becoming too weak to use. The beam angle is wider at low frequencies, meaning the outgoing pulse is spread out more and is better suited for viewing a larger area under the boat. However, this also means less target definition and separation and increased susceptibility to noise. Although low frequencies can see deeper, they may not give you a clear picture of the bottom.

A rule of thumb would be to use the 200 kHz setting for a detailed view to about 200 feet and then switch to 50 kHz when you want to look deeper. Better yet, display both views side-by-side on a split screen for both perspectives.

Cone Angle

The transducer concentrates the transmitted sound into a beam. In theory, the emitted pulse radiates out like a cone, widening as it travels deeper.
For the scope of this discussion, however, the idea of a cone works just fine. The signal is strongest along the centerline of the cone and gradually diminishes as you move away from the center. Wider angles offer a larger view of the bottom, yet sacrifice resolution, since it spreads out the transmitters power. The narrower cone concentrates the transmitters power into a smaller viewable area. Cone angles are wider at low frequencies and narrower at high frequencies.

To sum up, a wide cone angle can detect fish around the boat and not just those directly under it while exhibiting less target separation. A narrow cone concentrates the sound output enabling it to better detect small details, such as fish or bottom structure, but only scans a small amount of water at a time.